Steel frames are great but as tubing has become thinner, they can rot through alarmingly quickly. A few capfuls of motor oil are a good general precaution but not nearly enough for very thin tubes. Frame saver comes in a surprisingly small aerosol which promises to coat several frames. First off, wear old clothes, cover exposed areas of skin and find somewhere with good ventilation and ensure you have good access to a rag to clean off any spillages.
Shake the can for a minute or so and with the seat post removed spray a liberal blast into the frame and then and rotate it in the air. This is where frame saver caught me unawares, racing through the tubes. I hadn't plugged a breathe hole sufficiently and droplets of the fluid splashed onto the sleeve of an old shirt. Within a few seconds I felt a nasty burning sensation against the skin.
Do not be tempted to insert the straw into the tubes as this pressure build up can cause the liquid to spray back from the hole- you only have one pair of eyes!
Having given a liberal application, it's best the holes are plugged and then, if possible, the frame left to cure overnight. Ceeway suggest machines in heavy service should be given periodic reapplication via the seat tube just to be sure. When applied to a 'virgin' frameset, it cures to a golden honey colour similar to that of fresh motor oil. However, it did react with my older frame with a white powdery residue when the Framesaver dried. Whilst it works best on an untreated frame, it can be applied directly to frames, which have had previous treatments and oil or other automotive corrosion inhibitors inside them.
£10 is a small price for peace of mind, 125g will service a few frames and is a lot cheaper than having tubes replaced because it's rusted through.